1. Field of the Invention
The present invention relates to a joined body, particularly to a high pressure discharge lamp using a ceramic discharge tube.
2. Description of the Related Art
A high pressure discharge lamp has a ceramic discharge tube with two end portions, in which sealing members (usually referred to as a ceramic plug) are inserted, respectively, to seal the respective end portions. A through hole is formed in each sealing member and a metal member, to which a predetermined electrode system is attached, is inserted within the through hole. An ionizable light-emitting material is introduced and sealed within the inner space of the discharge tube. Known high pressure discharge lamps include a high pressure sodium vapor and metal halide lamps, the latter exhibiting more superior color coordination. The lamp may be used in high temperature condition by forming the discharge tube by a ceramic material.
In such discharge lamp, it is necessary to air-tightly seal between the end portion of the ceramic discharge tube and a member for supporting an electrode system. The ceramic discharge tube has a main body with a shape of a tube with two narrow ends, or a barrel, or a straight tube. The ceramic discharge tube is made of, for example, alumina sintered body.
The specification of Japanese Patent Application No. 178,415/1999 (EPO EP0982278, A1) discloses the following structure. The joining portion between the end portion of a ceramic discharge tube and a member for supporting an electrode system comprises main phase contacting the discharge tube, and an intermediate ceramic layer contacting the supporting member and existing between the supporting member and the main phase. The main phase is composed of a porous bone structure, with open pores and made of a sintered product of metal powder, and ceramic phase impregnated into the open pores in the porous bone structure. Herewith, such a joined structure has improved air-tightness and resistance to corrosion, and repeated thermal cycles do not result in the fracture of the joined structure.